One common application for thermal sensors is in thermal (infrared) detection devices such as night vision equipment. One such class of thermal detection devices includes a focal plane array of infrared detector elements or thermal sensors coupled to an integrated circuit substrate with a corresponding number of contact pads disposed between the focal plane array and the integrated circuit substrate. The thermal sensors typically define the respective picture elements or pixels of the resulting thermal image.
One type of thermal sensor includes a thermal sensitive element formed from pyroelectric material that exhibits a state of electrical polarization and capacitance dependant upon temperature changes in response to thermal radiation. Barium strontium titanate (BST) is one example of such pyroelectric material. For some applications, an infrared absorber and common electrode assembly may be disposed on one side of the thermal sensitive elements. A sensor signal electrode may be disposed on the opposite side of each thermal sensitive element. The infrared absorber and common electrode assembly typically extends across the surface of the associated focal plane array and may be coupled to each of the thermal sensitive elements. Each thermal sensitive element may have its own separate signal sensor electrode. Each infrared detector element or thermal sensor is defined (in part) by the infrared absorber and common electrode assembly and a respective sensor signal electrode. The thermal sensitive elements may function as a dielectric disposed between the common electrode and the respective sensor signal electrodes which function as capacitive plates.
Various techniques associated with fabrication of very large scale integrated circuits have been used to fabricate a focal plane array and its associated integrated circuit substrate. Examples of such techniques include wet etching and dry etching. Such dry etching techniques typically include ion milling, plasma etching, reactive ion etching, and reactive ion beam etching. Also, laser etching or milling techniques have been used for some applications. Both isotropic and anisotropic etching techniques may be used depending upon the desired configuration for the resulting focal plane array and/or its associated integrated circuit substrate.
Problems associated with both wet and dry etching techniques include undesired etching of the mask layer and/or undesired etching of the substrate and/or other layers deposited during previous fabrication steps. Also, techniques for determining the desired end point of an etching process are critical to prevent over etching and to increase both throughput and reproduceability of the fabrication process. Previously available etching techniques have not provided the desired selectivity to effectively fabricate a focal plane array with the desired throughput and reproduceability between product runs. Ion milling and laser milling techniques are often unsatisfactory because it is difficult to find a relatively thin but effective etch stop to prevent both techniques from removing more material than desired. Also, laser milling frequently produces slag which further limits the use of this technique.